1. Field of the Invention
The present invention relates to a core ball, a solder paste, a formed-solder, a flux-coated core ball and a solder joint that are designed to reduce a junction temperature with less alpha dose.
2. Description of Related Art
Recently, along development of compact information equipment, electronic components to be mounted on them have been downsized rapidly. A ball grid alley (hereinafter referred to as “BGA”) having electrodes at its rear surface is applied to such electronic components in order to satisfy a requirement of a narrowed connection terminal and a reduced mounting area because of the downsizing requirement.
In the electronic components wherein the BGA is applied to its semiconductor package, a semiconductor chip having electrodes is sealed with resin and solder bumps are formed on the electrodes of the semiconductor chip. The solder bump is that a solder ball is joined to the electrode of the semiconductor chip. This solder bump is connected to a conductive land of a printed circuit board so as to be mounted on the printed circuit board.
Recently, a three-dimensional high-density package is studied by stacking up the semiconductor packages in a height direction in order to meet the additional high-density mounting.
In a case that the BGA is applied to the semiconductor package for the three-dimensional high-density mounting, the solder ball may be crushed by a semiconductor package's weight. If such an accident happens, the solder may be forced out so that a circulatory shunt may happen between the electrodes.
In order to resolve the above-described problems, it is considered to use a ball whose hardness is higher than that of the solder ball. As the ball having a higher hardness, a solder bump using a Cu ball or Cu core ball is reviewed. The Cu core ball is that a solder coating (solder plating film) is formed on a surface of the Cu ball.
Since the Cu ball and the Cu core ball do not melt at a melting temperature of the solder, the solder bump does not crush at a mounting process even if the solder bump receives the weight of the semiconductor package. Therefore, the semiconductor package can be supported very well. Such technology associated to the Cu ball and the like is disclosed, for example, in International Patent Publication No. WO/1995/24113 (hereinafter referred to as “WO/1995/24113 publication”).
By the way, the downsizing of the electronic components allows the high-density mounting, but the high-density mounting causes soft error problems. The soft error is a possibility of rewriting a storage content of a memory cell in a semiconductor integrated circuit (IC circuit) with alpha ray entering into the memory cell.
It is conceivable that beta decay occurs on a radioactive element such as U, Th, Po, etc. in the solder alloy or a radioactive isotope included in Pb, Bi, etc. and then alpha decay occurs so that the alpha ray is emitted. Recently, a low alpha ray solder material is developed wherein the contained amount of the radioactive element is reduced. An associated technology is disclosed, for example, in Japanese Patent No. 4,472,752 (hereinafter referred to as “JP/4,472,752 patent”).
International Patent Publication No. WO/2013/14166 (hereinafter referred to as “WO/2013/14166 publication”) discloses a technology of a low temperature reflow performed with a low temperature melting point by using a core ball wherein a surface of a solder ball is coated with a plating layer comprising a Sn—Bi alloy. Japanese Patent Application Publication No. 2013-185214 (hereinafter referred to as “JP/2013-185214 publication”) discloses a technology that reduces the alpha dose of Bi as a solder material to a value equal to or less than 0.0100 cph/cm2. Japanese Patent Application Publication No. 2007-46087 (hereinafter referred to as “JP/2007-46087 publication”) discloses a technology that strengthens a joint strength and reduces the melting temperature.
By the way, the WO/1995/24113 publication discloses the Cu ball and the Cu core ball having the higher sphericity. However, this publication counts out the lowering alpha dose of the Cu core ball.
The WO/1995/24113 publication describes, in its background explanation column, only Pb—Sn alloy as the solder alloy of making up the solder film. The alpha ray is emitted from an isotope 210Pb of Pb included as an impurity in Sn material along the decay process 210Pb->210Bi->210Po->206Pb.
It is conceivable that the Pb—Sn solder alloy contains its radioactive isotope 210Pb because the Pb—Sn solder alloy contains Pb in large quantities wherein this solder alloy is only one material disclosed in the WO/1995/24113 publication. Therefore, even if this solder alloy is used as the solder film of the Cu core ball, it is impossible to reduce the alpha dose.
The WO/1995/24113 publication does not disclose a Sn plating on the Cu ball and an electrolytic plating under a flowage condition of the Cu ball and electrolytic solution.
In electrolytic refining described in the WO/1995/24113 publication, since an electrolytic deposition surface is limited to one direction, it is impossible to forma plating film having an even film thickness on a micro work piece such as the Cu ball.
The JP/4,472,752 patent discloses a technology of Sn ingot having a low alpha dose wherein it does not only electrolytically refine but also reduces the alpha dose by suspending adsorbent in the electrolytic solution to adsorb Pb and Bi.
According to the JP/4,472,752 patent, since a standard electrode potential of Pb or Bi is close to that of Sn, it is difficult to reduce the alpha dose only by depositing electrolytically Sn on a plate electrode with a general electrolytic refining. Assuming that the electrolytic refining is applied to a process of forming the plating film on the Cu ball and the adsorbent is suspended in the plating solution for the barrel plating similarly to the description of this patent, the adsorbent is stirred when stirring the plating solution and the work piece. According to this stirring, there is a possibility that the Pb ion and Bi ion absorbed on the adsorbent become carriers and are incorporated into the solder film along with the adsorbent.
The solder film, which takes the adsorbent, emits the high alpha ray. Since the adsorbent has a very small grain diameter of the order of sub-micron, it is difficultly conceivable to separate and collect the adsorbent after the suspension while flowing the plating solution. Therefore, it is difficult to prevent the adsorbent, which has absorbed Pb and Bi, from being incorporated into the film.
In addition, the WO/1995/24113 publication discloses a Pb—Sn based solder alloy. However, since this publication describes a plating method, a deposit method, a brazing method and the like as an equivalent method, it does not disclose a reduction of the alpha dose for a Sn based solder.
The subject matter of the WO/1995/24113 publication is to manufacture the Cu core ball having the high sphericity. On the other hand, the JP/4,472,752 patent discloses the reduction of the alpha dose by removing Pb from Sn material wherever possible in the electrolytic refining process.
Therefore, a person skilled in the art, who knows the WO/1995/24113 publication, cannot suppose a problem where it is required to reduce the alpha dose from the Cu core ball disclosed in this publication. In addition, the composition of the Pb—Sn solder is different from that of the Sn based solder. Thus, it is conceivable that huge numbers of trial-and-error processes are required to suppose the problem of reducing the alpha dose and to select the Sn based solder, especially the Sn—Bi based alloy solder from an endless number of solder alloys instead of the Pb—Sn solder alloy for the solder film.
It may be very difficult for those skilled in the art to make the plating solution by using the Sn ingot having the lower alpha dose described in the WO/1995/24113 publication and to form the Cu core ball with the plating method described in this publication.
If a joint is formed by using the Cu core ball manufactured with the prior arts described in the WO/1995/24113 publication or the JP/4,472,752 patent, there is a high possibility that radioactive elements in the solder film of the Cu core ball diffuse into the electrodes of the joint and then the alpha ray is emitted. Therefore, it is impossible to solve the soft error as a new problem of the high-density mounting.
The WO/2013/14166 publication discloses a technology that uses a Sn—Bi alloy as a plating layer (solder layer) and sets a contained amount of Bi in the alloy to 45-65% for allowing a reflow process under a low temperature equal to or lower than 160° C. However, this publication does not discuss what kinds of balls are used as the core ball and how to reduce the alpha dose of the Cu ball and the solder layer in order to countermeasure the soft error.
The JP/2013-185214 publication discusses a technology that prevents the soft error by suppressing the alpha dose of Bi used as the solder material. The subject matter of this publication is to improve only the solder material. However, it does not disclose at all a technology that attempts to reduce the alpha dose of the ball as the core in the core ball for reducing the alpha dose of the solder plating film.
The JP/2013-185214 publication describes the Sn—Bi alloy having the low alpha dose. However, this publication describes that an additive agent is not used for the alloy. It does not review at all the electrolytic plating that requires a usage of the additive agent, such as a chelating agent, a brightening agent and the like. As described above, although the Sn—Bi alloy is used by melting the pure metals of Sn and Bi in this publication, it does not completely the technology for forming the core ball with the electrolytic plating method (wet type plating method). In addition, since Bi decays to its radioactive isotope, it is difficult to reduce the alpha dose. As a result, the alpha dose of the manufactured Bi is equal to or less than 0.0100 cph/cm2 in this publication.
The JP/2007-46087 publication discloses a technology that forms a plating layer of the Sn—Bi alloy on the surface of the core ball made of Cu for increasing the joint strength and lowering the melting temperature.
However, for this purpose, a composition ratio of Bi contained in the plating layer must be changed from its inner portion (core surface) to its outer portion (outer surface) and thereby a technically difficult process is required. Moreover, this publication does not disclose a technique of lowering the alpha dose of the core ball as well as the solder plating film.